Superconductive microwave filter

ABSTRACT

Superconductive multiresonator microwave filter of strip transmission line structure housed in a cryostat. It comprises ground plates made of a superconductive metal, dielectric sheets bonded to said ground plates, a central metallic strip conductor made of superconductive material and bonded to said dielectric sheets, said ground plates, dielectric sheets and central conductor forming sections of strip-line, the resonance frequency of the resonators of the filter depending upon the lengths of said strip-line sections. Pads of dielectric material having a tapered end portion can slide in notches provided in the dielectric sheets thus forming sliding paths intersecting said strip-line sections. Control means are provided for remotely positioning the pads from the outside of said cryostat, thus allowing tuning of the resonators of the filter.

[ Dec. 24, 1974 Minet etal. v

[541 ISUPERCONDUCTIVE MICROWAVE FILTER I [76] Inventors: Roger P. Minet; Jean H. Debeau,

both of 60 Residence Corlay, 22300 Lannion; Ernest L.- Thepault, 2 Rue du Docteur Roux, 22700 Perros-Guirec, all of France 22 Filed: Feb. 20, 1974 2i Appl.No.:444,103

[58] Field- 0i Search..;. 333/73 R, 73 S, 84 M, 84 R, 333/73 C, 98 S, 97 R, 82 R, 82 B I [56] References Cited UNITED STATES PATENTS 2.867.782 l/l959 Arditi .l 333/84 M X Primary Examiner-Archie R. Borchelt Assistant Examiner-Marvin Nussbaum Attorney, Agent, or Firr nAbraham A. Saffitz 57] ABSTRACT Superconductive multiresonator microwave filter of strip transmission line structure housed in a cryostat. It comprises ground plates made of a superconductive metal, dielectric sheets bonded to said ground plates, at central metallic strip conductor made of superconductive material and bonded to said dielectric sheets, said ground plates, dielectric sheets and central conductor forming sections of strip-line, the resonance frequency of the resonators of the filter depending upon the lengths of said strip-line sections. Pads of dielectric material having a tapered end portion can slide in notches provided in the dielectric sheets thus forming sliding paths intersecting said strip-line sections. Control means are provided for remotely positioning-the pads from the outside of said cryostat, thus allowing tuning of the resonators of the filter 4 Claims, 9 Drawing Figures PATEHTEU SE82 41974 SHEET 7 BF 7 FIGS 1 SUPERCONDUCTIVE MICROWAVE FILTER The present invention generally relates to superconductive microwave filters and more particularly to superconductive multicell filters of the so-called capacitive gap-coupled strip transmission line structure and cross-cell strip transmission line structure.

These capacitive gap-coupled strip transmission line filters are disclosed at page 440 and so on of textbook: MICROWAVE FILTERS, IMPEDANCE- MATCHING NETWORK AND COUPLING STRUC- TURES by Messrs. George L. MATTI-IAI, Leo YOUNG and E. M. T. JONES, published by McGraw- Hill Book Co. Cross-cell strip transmission filters are disclosed in our co-pending application Ser. No. 444,140 filed Feb. 20, 1974.

They comprise a main strip line and open circuit halfwave sections or stubs of a strip line that are perpendicular to the main strip line, equally. space incremented along the latter, so forming crossestherewith. These sections are connected to the main line near, but not exactly, at their midpoint. Filter amplitude-frequency response is function of the distance between the connecting points of the main line and the stubs, and the midpoint of the stubs inthe cross cells of the filter.

. The invention will be now disclosed in detail with Insofar as concerns above-given microwave filters,

tuning of a filter cell to a given frequency requires that the length of each main strip line section in the case of capacitive filters or the length of two stub spurs Further means for adjusting the admittance of the strip transmission line sections forming the filter are provided and consist in dielectric tuning pads of the same material as that used for strip line dielectric sheet, with one end that is sawtooth shaped which are adapted to slide into grooves built into this dielectric sheet. The filter is set in a cryostat and the pads are controlled from outside the cryostat. vTheir path cuts over the strip transmission line sections so that when the pad is fully pulled out, the strip line has air as di' electric over a length equal to the pad rib width, while when the pad is fully pushed in, the strip line has as dielectric the dielectric used over a length equal to the pad rib width, and finally when the pad id partially in place, the ratio between the length of the portion where air is the dielectric and the length of the portion where the dielectric is the dielectric used varies with the pushin distance.

pertinent referrals to the accompanying drawings, wherein:

FIGS. 1a and 1b depict the structure of a nonsuperconductive cross cell microwave filter, with two cells according to prior state-of-the-art',

FIG. 2 depicts the structure of a nonsuperconductive"capacitive gap-coupled microwave filter according to prior state-of-the-art;

-- in the case of cross filters be defined with extremely high accuracy reaching up to ma. Such accuracy can be attained owing to special care given in line with thin film wafer techniques and chemical etching processes. Keeping in mind the uncertainties arising over the exact values of dielectric constants and creep due to cooling at liquid helium temperature (4.2 K), it is not possible to assume the admittance of a filter at the remperature of 4.2 K just because'of its admittance at an ambient temperature.

During filter cell cooling, dielectric sheets crimp heavily as well as their casing but within different con-' traction coefficients. If the casing is in'a metal having tightening of the two metal ground plates varies, and this leads to a variation in their spacing. Due to the fact that for a given dielectric material admittance of a strip line mainly varies in function of the ratio of the width of the metallic strip to the metallic ground plate spacing, the cooling process detunes the filter. Microwave filters built by the petitioners for a mid-band frequency of 4,000 MHz, using as a dielectric substance tetrafluorethylene and encased in brass exhibited a scatter,- ing of the mid-band frequencyof their resonators over a range of 150 MHz.

Scatter-free superconductive microwave filters are obtained through two combined arrangements. Outer casing plates, wafering over the superconductive ground plates on either side of the dielectric sheet of the strip line are in a contractable metal with a thermal expansion coefficient that is approximately equal to a relatively high thermal expansion coefficient, the

FIG. 3 represents an assembly of a capacitive gapcoupled filter housed in a cryostat along with its tuning mechanism;

FIG. 4 is a blown up representation of the capacitive gap-coupled filter tuning mechanism; I FIG. 5 shows an assembly of a cross cell filter housed in a cryostat along with its tuning mechanism;

FIG. 6 is a blown up representation of the cross cell filter tuning mechanism;

FIG. 7 is an exploded view of a superconductive capacitive gap-coupled microwave filter structure;

FIG. 8 is an exploded view'of a-three-cell cross cell filter structure.

FIGS. la and lb show a microwave filter with two cross cells, according to the prior art. It comprisesa main strip transmission line with a metallic strip conductor l bonded to dielectric sheets 2 and 3 to the other side of which are bonded metallic ground plates 4 and 5. The two ground plates are electrically interconnected by means that are not shown in FIGS. la and lb. Each filter cell comprises a pair of arms or stubs, 6 and 7 for the first cell and 106 and 107 for the second; these are located on either sides of the strip transmission line 1 and parallel-connected to the same. Each of these stubs is open-circuited and approximately a quarter wavelength long,-while the two aligned stubs of a same pair form a section of strip line which is exactly one half wavelength'long. The two stubs of a same pair do not exactly the same length; in other words, the strip line half-wave sections formed by stub pairs are not exactly connected at their mid-point in relation to the main strip transmission line. The respective lengths 1 and 1 of stubs 6 and 7, and 106 and 107, are such that:

where 1,, is the quarter wavelength and e is a'small coefficient, much lower than unity and different for each stub pair.

lium cryostat and 11 is such cryostat cover. The filter and the tuning mechanism are both fitted into a casing 12, held by a low-diameter tube 13, in which the two coaxial signal input/output cables 90,, 90 go through. Inner coaxial cable leads are shown as 82. A cylindrical tank 14 is fastened to tube 13, to astrain the liquid helium poured in through the filling aperture 15 within a set annular volume. This tank is used to isolate the boiling helium from the filter which, if it were placed in direct contact with the boiling helium, would exhibit filter frequency response curve instabilities.

Four rods, 16, to 16,, cross through the still tightproof cover 11. They control the filter tuning pads and are tipped within the cryostat by rev-counter buttons, 17, to 17,. These rev-counter buttons display over a dial housed. in the button the rev-count which they were subjected to from the initialstarting position, as is known in prior state-of-the-art.

The cryostat, the tank, the casing and these tube and rods could, for instance, be in stainless steel.

Stub end 13 is fastened to a vertical plate 18, through a link part 19 (FIG. 4). This plate is depicted as partially exploded on FIG. 4 to enable seeing some tuning mechanism components.

The capacitive gap-coupled filter casing 12 is fastened at plate 18 bottom area. Slots 20 are fitted into plate 18 to guide four mobile yokes, 21, to 21 each bearing a tuning pad 71-74. Only three of these pads are seen on FIG. 4, 71, 72, 73.

Yoke 21, to 21,, vertical travel is controlled from the outside of the cryostat by means of rods 16, to 16 The lower tip of rods 16, to 16., is threaded at 23 and the threaded part ofeach rodfits within a threaded hole 24, made into each of the yokes. Gear play is cancelled out by spring 25.

Coaxial cables 26, and 26 reaching the filter go through tube 13.

The three-cell cross cell superconductive filter tuning system is shown on FIG. 5. This figure is similar to FIG. 3, although with six tuning rods 26 to 26 each ending with a rev-counter button 27, to 27 respectively. Reference numerals are the same for FIGS. 3 and 5.

FIG. 6 depicts a cross cell microwave superconductive filter tuning mechanism.

The end of tube 13 is fastened to a vertical plate 28 by means of a link part 29 (FIG. 6). This plate is shown partially exploded in order to bring out some tuning mechanism part into view.

The casing 32 of the cross cell filter is fastened to plate 28, toward mid-point. Plate 28 is used as a guide to two crosspieces 30 and 31, each longitudinally ribbed and in which the plate slides. The travel of crosspiece 30 is controlled by threaded rod 34 and'that of crosspiece 31 is controlled by threaded rod 35. On the one hand, each crosspiece is interlocked with a pad 36 or 37 respectively and, on the other hand, is used as a slide for a yoke bearing another pad 38 or 39 respectively. This other pad is controlled by rod 40, insofar as concerns pad 38, and by rod'41, insofar as concerns pad 39. As shown on FIG. 8, pads 36-38 and 37-39 have for purpose the tuning of the three-cell filter extreme cells.

1 through its top horizontal edge while pads 37-39 enter through its lower horizontal edge and pads 43 and 44 each enter through a vertical edge.

We now refer to FIG. 7 and see that casing 12 is built of two plates 51 and 52, plus two U-shaped spacers, with a rectangular section, 53 and 54. These pieces 51-54 are in high-expansion alloy, for instance an aluminium alloy. The thickness of plates 51 and 52 is so thinned out as to form a hollow recess at plate ends, used as a base for the spacers. Between the outer plates, verylow expansion plates 55 and 56, superconductive plates 57 and 58, dielectric sheets 59 and 60 are stacked up. Plates of the same type are symmetrically stacked in relation to the casing median plane. As an example, plates 55 and 56 are in invar, plates 57 and 58 in lead or niobium, sheets 59 and 60 in tetrafluorethylene or in Rexolite 1422.

The main strip line 69 of the capacitive gapcoupled filter is metallized over plate 59. This strip line is made of metallized lead. Instead of being aligned as depicted on FIG. 3, the strip line elements are right-angle bent to form an open rectangle at the access side. FIG. 7 gives the mm length of elements and of gaps in the case of a filter with a center frequency of 4,000 MHZ, a bandwidth of 6 MHz and a ripple amplitude of 1.3 dB. FIG. 7' also shows tuning pads 71-74 layout. Pads 71-72 go through holes 61 and 62 made into spacer 53, while pads 73-74 go through holes 63 and 64 made into spacer 54. Notches 65 and 66 are provided in spacer 53, to house coaxial-strip line transitions. Strip line connectors 67 and 68 are fitted on the internal side of spacer 53, in order to connect the main strip line 69 to the strip line side of the connectors. Sheet 60 is notched at its lower facing to provide recesses for the dielectric tuning pads 71-74.

Turning now to FIG. 8, casing 32 is similar to casing 12, except for spacers 53 and 54, sheet 60 and conductor 69 which become respectively 83, 84, and 89. Instead of four dielectric pads 71-74, there are six dielectroc pads 36-39 and 43-44 and their layouts are shown in FIG. 8. Spacings between pads 36-38 and 37-39 respectively are equal, while spacings between pads 71-72 and 73-74 respectively are unequal. Additional recesses are provided on the lower facing of sheet 80 to house pads 43-44.

Pads 36-38 go through holes 91-92 made into spacer 83 while pads 37-39 go through holes 93-94 into spacer'84. A notch 85 and a notch 86 are provided respectively in spacers 83 and 84 to house coaxial strip line transitions. Strip line connectors 87 and 88 are provided respectively in spacers 83 and 84 in order to connect the main strip line 89 to the strip line side of the connectors.

What we slain! is; i a a 1. A superconductive multiresonator microwave filter of strip transmission line structure housed in a cryostat comprising ground plates made of a superconductive metal, dielectric sheets bonded to said ground plates, a central metallic strip conductor bonded to said dielectric sheets, said ground plates, dielectric sheets and central conductor forming sections of strip-line, the resonance frequency of the resonators of the filter depending upon the lengths of said strip-line sections, pads of dielectric material having a tapered end portion, notches in said dielectric sheets forming sliding paths for said pads intersecting said strip-line sections and control means for remotely positioning said pads from the outside of said cryostat.

2 A superconductive multiresonator'microwave filter of strip transmission line structure housed in a cryostat comprising cover plates made of a material having a relatively large coefficient of thermal expansion, intermediate plates made of a material having a relatively small coefficient of thermal expansion, ground plates made of a superconductive metal, dielectric sheets made of a dielectric material having a coefficient of thermal expansion near the thermal expansion coefficient of said cover plates and bonded to said ground plates, a central metallic strip conductor bonded to said dielectric sheets, said ground plates, dielectric sheets and central conductor forming sections of strip-line, the resonance frequency of the resonators of the filter depending upon the length of said strip-line sections, pads of dielectric material having a tapered end portion, notches in said dielectric sheets forming sliding paths for said pads intersecting said strip-line sections and control means for remotely positioning said pads from the outside of said cryostat.

3. A superconductive multiresonator microwave filter of strip transmission line structure as claimed in claim 1, wherein the central conductor has the general shape of a rectangle and comprises sections of the periphery of said rectangle having a length approximately equal to a half wave length at the mid frequency of the filter and coupled therebetween by end capacitive gaps, the ground plates, the dielectric sheets and said central conductor forming sections of a strip-line defining a capacitor gap-coupled filter.

4. A superconductive multiresonator microwave filter of strip transmission line structure as claimed in claim 1, wherein the central conductor has the general shape of a main rectilinear line and of a plurality of line stubs having a length approximately equal to a half wave length at the mid frequency of the filter and connected to said main rectilinear line near the middle of the said stubs at points of the main line equallyspaced apart, the ground plates, the dielectric sheets and said central conductor forming sections of strip-line defining a cross cell filter. 

1. A superconductive multiresonator microwave filter of strip transmission line structure housed in a cryostat comprising ground plates made of a superconductive metal, dielectric sheets bonded to said ground plates, a central metallic strip conductor bonded to said dielectric sheets, said ground plates, dielectric sheets and central conductor forming sections of strip-line, the resonance frequency of the resonators of the filter depending upon the lengths of said strip-line sections, pads of dielectric material having a tapered end portion, notches in said dielectric sheets forming sliding paths for said pads intersecting said strip-line sections and control means for remotely positioning said pads from the outside of said cryostat.
 2. A superconductive mulTiresonator microwave filter of strip transmission line structure housed in a cryostat comprising cover plates made of a material having a relatively large coefficient of thermal expansion, intermediate plates made of a material having a relatively small coefficient of thermal expansion, ground plates made of a superconductive metal, dielectric sheets made of a dielectric material having a coefficient of thermal expansion near the thermal expansion coefficient of said cover plates and bonded to said ground plates, a central metallic strip conductor bonded to said dielectric sheets, said ground plates, dielectric sheets and central conductor forming sections of strip-line, the resonance frequency of the resonators of the filter depending upon the length of said strip-line sections, pads of dielectric material having a tapered end portion, notches in said dielectric sheets forming sliding paths for said pads intersecting said strip-line sections and control means for remotely positioning said pads from the outside of said cryostat.
 3. A superconductive multiresonator microwave filter of strip transmission line structure as claimed in claim 1, wherein the central conductor has the general shape of a rectangle and comprises sections of the periphery of said rectangle having a length approximately equal to a half wave length at the mid frequency of the filter and coupled therebetween by end capacitive gaps, the ground plates, the dielectric sheets and said central conductor forming sections of a strip-line defining a capacitor gap-coupled filter.
 4. A superconductive multiresonator microwave filter of strip transmission line structure as claimed in claim 1, wherein the central conductor has the general shape of a main rectilinear line and of a plurality of line stubs having a length approximately equal to a half wave length at the mid frequency of the filter and connected to said main rectilinear line near the middle of the said stubs at points of the main line equally spaced apart, the ground plates, the dielectric sheets and said central conductor forming sections of strip-line defining a cross cell filter. 